I'm heartbroken to inform you that a hat like that does not exist. I thought surely someone must have made a hat that just simply said "ASS" on it. But alas, I was wrong. There seems to exist no hat that simply says "ASS" on it. You could get a hat that has custom text on it, but let's not fool ourselves. It wouldn't be the same.
So I guess, in the end, you could say... An ass like that does not exist.